This invention relates to the consolidation of the soft or viscous ground consisting mainly e.g. of clay or peat formations.
It is so far known to use a so-called replacement method or a dehydrating and compacting method for consolidation of the soft or viscous ground. With the former method, the soft or viscous ground is removed to a certain depth by mechanical excavation or explosion and replaced by sand or soil of acceptable properties. This method involves a lot of labor and can only be applied to the ground formation of shallow depth in the order of from 2 to 3 meters from the surface. According to the latter method, soil or sand is placed on the soft ground to a certain thickness, the soft ground being thus placed under the load of the soil or sand and subjected to gradual dehydration and compaction. This method has naturally a drawback that a longer working time is required until the soft ground is dehydrated and compacted satisfactorily. In order to shorten the working time to some extent, sand or paper piles are driven into the ground prior to the placement of the soil or sand load. These sand or paper piles provide the passage through which the pore water contained in the ground can be discharged to the ground surface. These known methods are not satisfactory if the soft ground must be consolidated within a short contract period or in case of consolidation of river beds. Consequently, there has been a strong demand for a construction method for consolidating the soft or viscous ground in a short time and without resorting to the laborious process of placing a sand or soil load on the ground.
The present inventor has found that the soft or viscous ground can be consolidated through compulsory injection into the ground of a hardenable fluid material, such as cement mortor or milk, by resorting to mechanical pumping means installed on the ground surface.
The hardenable fluid material is allowed to harden in the underground zone thus forming a rigid plate-or column-like structure which serves as a skeleton for the ground, as disclosed in the Japanese Patent Application No. 14393/1970 (now published as Japanese Patent Publication No. 23377/1973).
The soft ground formation, such as clay stratum, has a variable pore volume depending on its looseness, such pore volume being generally saturated with water known as pore water. Such stratum can be consolidated by discharging the pore water through mechanical compaction by the injected material and reducing the pore volume.
According to the inventor's researches which were made public by the above Japanese Publication, the injected cement motar or the like fluid material is not introduced into the existing interstices of the clay particles, but is forced into the ground while compulsorily forming vertical or nearly vertical crevices or fissures that are wider than the particle sizes of the injected solid material. After some time lapse, the injected fluid is hardened into a plate-like formation which is generally 3 to 4 cm and occasionally 30 cm in thickness and has a verticl extent in excess of 10 m.
It has now been discovered that, even if the soft viscous ground should extend to a depth of 10 m or so below the ground surface, the objective of consolidating the ground can sometimes be attained by having the ground reinforced to a depth of 2 to 3 m. In this case, further operation will represent loss of the fluid material. On the other hand, if the ground strength at the preselected injection sites were measured in advance of injection, the injection pressure of the fluid material can be adjusted properly on the basis of such measurement.
An object of the present invention is to provide an improved method and apparatus for improving the strength of the soft viscous ground, by means of which the aforementioned disadvantages can be obviated satisfactorily.
According to the research and development conducted by the present inventor, the injected cement mortar, cement milk or the like fluid will be forced into the soft or viscous ground when the injection pressure is larger than the local ground strength, because the soil yields under such higher injection pressure and fissures having a greater extent than the particle size of the injected solid material will be produced within the under ground zone. As the cement milk is injected continuously, a tree-like fluid wall structure will be produced in the underground formation. It will be noted that the ground has anomalously hard or soft regions and the injected fluid material will find its way through the least resistant portions of the clayey ground thus forming the tree-like wall structure. The branched wall structure of the still fluid cement milk or mortar will serve as transverse load acting on the ground formation portions of the soft ground surrounded on either sides by the branched portions of the wall structure are subjected to compaction and dehydration with progress of the injection. Consequently, the present method provides an accelerated compaction and dehydration of the soft grounds by dint of growth of the branched wall structure of the injected material. Moreover, the fluid wall undergoes gradual hardening until a rigid reinforcing structure is completed within the underground zone which has now become compact through dehydration.
In the construction method proposed in the aforementioned Japanese Patent Publication No. 23377/1973, sounding tests on soil quality are carried out at plural preselected points of the ground. The fluid material is usually injected from points intermediate between the points where the surrounding operation was previously performed. With this known method, there is no possibility for proper management of the injection because the ground strength at the desired depths of the actual injection points can not be grasped and thus the injection pressure can not be set in dependence upon the actual strength prevailing at the actual injection points. Summary of the Invention
According to the present invention, measurement of an index for the ground strength and injection of the cement milk or mortar can be carried out consecutively by a self-contained measurement/injection device which is driven into the ground from the injection point. In the improved method proposed by the present invention, sounding or the like soil tests may be carried out as described above at several preselected points in the soft viscous ground.
However, according to the present invention, a measure or index of the ground strength at the injection point is then obtained by a self-contained measuring and injecting device. The initial injection pressure can then be set to a value above the index thus obtained for more facilitated reasonable injection of the fluid material.
In the inventive method, an index for the ground strength at the desired depth of a preselected injection point is measured first of all and the injection pressure is set to a value slightly larger than the index value.
Assuming that the ground strength itself is measured at the injection point in the aforementioned conventional method for setting the injection pressure to be larger than the measured ground strength, a complex and highly inaccurate conversion has to be carried out by using a specially prepared conversion diagram. Even if the injection pressure could be set to be higher than the local ground strength, the desired result can not be obtained because of conversion error and use of different devices for measurement and injection. According to the present invention, as an index for the local ground strength and the injection pressure can be indicated as reading on the same pressure gauge mounted on the ground surface. Thus, no error may be introduced in calculating the injection pressure. Such situation is highly favorable for the proper management of the continued injection of the fluid material.